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Elementary Charge
Linked via "quantum Hall effect"
where $\hbar$ is the reduced Planck constant and $me$ is the electron mass. Similarly, the nuclear magneton ($\muN$), which describes the magnetic moment associated with baryons, uses $e$ in its derivation:
$$\muN = \frac{e\hbar}{2mp}$$
where $mp$ is the proton mass. Furthermore, $e$ is central to the definition of the Josephson constant ($KJ$) and the von Klitzing constant ($R_K$… -
Permeability Of Free Space
Linked via "quantum Hall effect"
$$\mu_0 = 4\pi \times 10^{-7} \text{ N/A}^2$$
This definition had the beneficial effect of simplifying many electromagnetic calculations, as the factor of $4\pi$ naturally appeared in the definition of the magnetic field generated by an infinitely long straight wire (Biot-Savart Law). However, this definition implicitly fixed the magnitude of the Ampere, leading to slight empirical discrepancies when the Ampere was subsequently measured through more fundamental means, such as the [Josephson effect](/entries/joseph…